The invention relates to a corpuscular beam device with an objective lens for focussing a primary particle beam onto a specimen and a detector for detecting the secondary and/or backscattered particles released at the specimen.
High resolution objective lenses for image generation of a specimen comprise electrostatic retarding field lenses or combined electrostatic magnetic lenses. Such lenses use a higher beam energy in front of the lens and decelerate the beam to the lower final beam energy inside the objective lens. The deceleration for the primary particle beam is also used for the extraction and the acceleration of the secondary particles released at the specimen. The secondary and/or backscattered particles are transferred to detection means for registration and for forming an image of the surface of the specimen.
In high resolution lenses a high immersion ratio (Ein:Eout) is applied, since a high immersion ratio reduces the aberration of the lens, especially the spherical and the chromatic aberration. Since the retarding field is arranged in the objective lens close to the specimen, a high immersion ratio also means in conventional arrangements a strong extraction field for the secondary particles. This results in a good collection efficiency, but also generates problems when imaging insulating specimens. A strong extraction field results in a strong positive charging of the insulating surface, which makes imaging impossible or at least disrupts the imaging significantly.
To overcome this problem EP-A-0 333 018 proposes an objective lens with an additional control electrode arranged above the specimen. By applying a voltage to this control electrode, an additional field can be superimposed on the xe2x80x9cleakage fieldxe2x80x9d of the objective lens. Correspondingly, the field generated by the objective lens will be influenced and any field strength required at the specimen can be adjusted. However, if the specimen or the column is tilted, the control voltage of the control electrode causes an asymmetrical field between the specimen and the lens. This asymmetrical field affects both the primary beam (slight deflection) and the secondary beam (tilted secondary electron distribution with effects on detection efficiency).
The object of the invention, therefore, is to improve the corpuscular beam device according to the preamble to claim 1 so that it has a high electrical field inside the objective lens, a low electrical extraction field in the region of the specimen and a high secondary and/or backscattered particle detection efficiency.
This object is achieved by the features of claim 1 in that the electrode arranged above the specimen is a sieve electrode with a central hole for the primary particle beam and a plurality of additional holes for the secondary and/or backscattered particles. If the diameter of this central hole is small enough, e.g. less than 2 mm, the leakage field of the objective lens is small. By such a design surface charging can be avoided totally. Since the additional holes of the sieve electrode are also small, the extraction field can be kept small, but the detection probability using a larger number of holes can be increased significantly.